1. Field of the Invention
The present invention relates to apparatus for monitoring steam and ethylene oxide sterilization cycles.
2. Description of the Prior Art
A variety of means are available for monitoring the efficacy of sterilization cycles. Standardized spores of a strain sufficiently resistant to the sterilization medium are placed on a substrate and are exposed to the sterilization process. Sterilization of the standardized spore strain provides a high degree of confidence that sterilization of bacterial strains in the chamber load has occurred. Conversely, survival of the standardized spore stain indicates unsatisfactory sterilization of the load. The Association for the Advancement of Medical Instrumentation (AAMI) has published recommendations for evaluating both ethylene oxide and steam sterilizers. Test packs containing biological indicators and other materials are intended to challenge all of the parameters necessary for ethylene oxide or steam sterilization. In actual chamber loads, it may take several minutes into the sterilization cycle before the load is sufficiently exposed to sterilant for sterilization of the load to occur. The test packs should not be sterilized prior to the sterilization of the load. One requirement of a test pack, therefore, is to impede the flow of sterilant to the biological indicator to more closely simulate the rate of sterilization experienced by the load.
Biological indicators are defined by AAMI as a calibration of microorganisms of high resistance to the mode of sterilization being monitored, placed in or on a substrate, packaged to maintain the integrity of the inoculated substrate in a manner convenient to the ultimate user, which serve to demonstrate that sterilization conditions were met. The foregoing definition of biological indicators shall be adopted for purposes of the instant application.
A biological indicator for monitoring the efficacy of ethylene oxide sterilization typically includes Bacillus subtilis spores. A biological indicator for monitoring the efficacy of steam sterilization typically includes Bacillus stearothermophilus spores. However, any microorganism which is highly resistant to the particular sterilization medium will suffice. Vegetative bacteria are known to be easily killed by steam and, thus, are not recommended. Thermotolerant organisms, such as Bacillus spores are known to be extremely resistant. The challenge organism should be more resistant to the sterilant and the population should be greater than the bioburden of the chamber load.
For qualification testing of an ethylene oxide sterilizer, AAMI recommends placing a biological indicator into the barrel of a plastic syringe so that the diaphragm of the plunger does not touch the indicator. The needle end of the syringe must be open. A plastic airway, a length of latex tubing and two such syringes are placed in the center of a stack of folded surgical towels. The stack is then wrapped in a wrapping material. The resulting test pack is designed to challenge the parameters of ethylene oxide sterilization. The towels act as moisture and ethylene oxide absorbents and as a heat sink, the latex and plastic tubes act as ethylene oxide absorbents, and the plastic syringes act as heat sinks, ethylene oxide absorbents and a means for impeding the flow of sterilant to the biological indicators. The biological indicators provide the microbial challenge.
The heat sinks are necessary to provide a thermal challenge to the sterilization process. The test pack and, particularly, the environment surrounding the biological indicator should simulate the thermal profile of the normal chamber load. The heat sinks act to slow down the transfer of heat to the spores. In the ethylene oxide application, the additional parameters of humidity and ethylene oxide permeation are equally, if not more, important.
Recommended means for routinely monitoring ethylene oxide sterilization is to wrap the syringe and biological indicator described above in a clean huck towel and seal them in a standard peel pouch. One commercially available monitor includes a plastic syringe which contains a biological indicator and a stack of filter paper discs within the barrel of the syringe. The discs are disposed between the open needle end of the syringe and the biological indicator. The biological indicator includes a plastic ampule with a cap. The cap has a hole in the center to permit the sterilant to enter. A vial of growth media and a spore strip are enclosed in the ampule. Following the sterilization cycle, the ampule is squeezed to break the vial to immerse the spore strip in growth media. The barrel of the syringe is enclosed in a fiber board tube. A chemical process indicator to indicate exposure to ethylene oxide gas is disposed on the fiber board tube. The syringe is placed in a standard peel pouch having a clear plastic sheet heat sealed to a medical grade paper backing which is permeable to the sterilant.
A variety of similar ethylene oxide monitors are commercially available. All are designed to monitor only ethylene oxide sterilization cycles.
A problem sometimes encountered in steam sterilization is air entrapment. Cool air pockets can insulate portions of the load preventing exposure to the sterilant. Very small amounts of air do not impede sterilization as long as the air and steam are thoroughly mixed. When steam contacts cooler objects, the objects absorb the latent heat of the steam, the steam collapses, condensate collects on the objects and any air present remains. Additional steam is drawn to the area and the process is repeated so that air is accumulated and compressed into cool air pockets. Various devices are available to detect critical quantities of air in steam sterilizers. The test packs used to monitor the efficacy of steam sterilization are not designed as air detectors but should be sensitive to air which impedes sterilization. The environment to which the biological indicators are exposed should simulate the sterilizer environment.
The AAMI recommendations for monitoring the efficacy of steam sterilization cycles include using a packaging material which allows adequate air removal to avoid the disproportionate accumulation of air around the biological indicator. The material must also allow steam penetration of the package contents and should provide an adequate barrier to microorganisms. The recommendations for a test pack for use in both gravity-displacement and prevacuum sterilizers include using an appropriate biological indicator in a 16 towel test pack. The towels are folded and stacked. The biological indicator should be placed between the 7th and 8th towels in the geometric center of the pack.
A commercially available test pack consists of a small clear plastic tube containing a biological indicator and a chemical process indicator strip. The tube is closed by two plastic caps. A hole at the end o each cap vents the tube and permits the entry of sterilant. Cotton filled gauze covered sponges inside the tube provide means for loosely maintaining the biological and chemical indicators in the midsection of the tube. Loose fitting water-repellent foam discs are disposed at each end of the tube adjacent to the holes in the caps. The dual vent system provided by the opposing holes in the caps is said to mimic the characteristic resistance of the AAMI recommended 14-towel test pack to removal of air by prevacuum or gravity-displacement sterilizer air removal methods. The disposable test pack described is designed for use with steam sterilizers only.
Another type of commercially available steam monitoring test pack, also adapted for use in monitoring only steam sterilization cycles, comprises a stack of filter paper. A cavity is cut out of the center of the stack to house a biological indicator. One manufacturer then wraps the stack of filter paper in a surgical grade paper wrap. Another places the stack in a box. Sterilant entry is from five sides of the stack of filter paper.
A problem with the AAMI recommended procedures is reproducibility from test pack to test pack due to the differences in the quality of towels used and the variance in technique of the persons preparing the test packs. The commercially prepared test packs attempt to standardize the test packs.
An object of the present invention is to provide a single, standardized test pack for use in monitoring the efficacy of both steam and ethylene oxide sterilization cycles. A further object of the present invention is to provide such a test pack which provides a monitor of both steam and ethylene oxide sterilization cycles having the same degree of challenge as the test packs recommenced by AAMI.